The present invention relates to an intake apparatus for an engine and, more particularly, to an intake apparatus for an engine in a vehicle which has, in an engine room, a cooling duct for introducing travel wind to a cooling device such as an intercooler.
An intake duct for supplying intake air to a combustion chamber is equipped in an engine. The diameter of the intake duct is determined with reference to the size of a throttle portion provided to the intake duct. For this reason, the intake duct has a relatively elongated shape.
Some engines with exhaust turbo chargers have, in their air intake systems, intercoolers for cooling intake air. As the intercooler, a technique described in Japanese Utility Model Laid-Open No. 63-58030 is known. As disclosed in this prior art, many intercoolers are of an air cooling type, which cools intake air heated by a turbo charger using travel wind introduced to it. In the case of an air cooling type intercooler, a duct for introducing travel wind is designed to have a larger path sectional area than that of an intake duct since it must introduce a large amount of air. In addition, the duct is arranged to effectively introduce travel wind by utilizing a dynamic pressure produced by traveling.
An intake duct is designed to have a relatively small diameter so that intake air is most efficiently supplied to a combustion chamber in a normal driving state. Such an intake duct cannot supply a necessary amount of air to an engine within a short response time especially when a high output is required like in an immediate acceleration state. In order to solve this problem, an auxiliary duct for introducing intake air only when a high output is required may be arranged in addition to a conventional intake duct. However, it is difficult to dispose such an auxiliary duct since an engine room has a limited space, in particular, since a bonnet tends to have a low profile in recent years.
In a known arrangement of the exhaust turbo charger described above, two turbo chargers, i.e., a low-load charger with a small A/R, and a high-load charger with a large A/R are arranged, as disclosed in, e.g., Japanese Patent Laid-Open No. 60-216031. In this prior art, intake air is compressed by the low-load charger in a low-load driving state. In a high-load driving state, intake air is compressed by both the low- and high-load chargers. Reference symbol A denotes a minimum area of an outlet pipe in a turbine, and reference symbol R denotes a distance from the center of rotation of the turbine to the central axis of the outlet pipe.
When the two turbo chargers are arranged, they are connected to air cleaners via independent low- and high-load intake paths, respectively. In general, the layout of these two intake paths is determined according to that of an engine and peripheral devices.
In a low-load state including an idling state before or after warming up of an engine, an intake air amount and a fuel supply amount are small, and a combustion property tends to be easily impaired. In addition, a requirement for a response time upon start of a vehicle becomes severer than in a high-load driving state. When the layout of the low- and high-load intake paths is determined according to that of the engine and its peripheral devices, as described above, a long low-load intake path must be arranged, and an intake response time is undesirably prolonged. In addition, the low-load intake path must be arranged on an upper portion of an engine room having a relatively low environmental temperature, and, hence, gasification and atomization of fuel are not accelerated, resulting in an impaired combustion property.